feat: Add staking functions (#19)

* add staking functions first commit

* fix nominate test and run linter

* use yarn docs

* move UnsignedTransaction into ./utils to enable generic tx construction

* update docs for UnsignedTransaction move

* rename TxInfo and TEST_TX_INFO to specify that they are for balance transfer

* move TxInfo into interfaces to reduce duplicate code

* make decode somewhat generic

* rename util/interfaces to util/types

* add encodings to tests